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REVIEW ARTICLE
Pseudoelasticity and thermoelasticity of nickeltitanium alloys: A clinically oriented review.
Part I: Temperature transitional ranges
Margherita Santoro, DDS, MA,a Olivier F. Nicolay, DDS, MS,b and Thomas J. Cangialosi, DDSc
New York, NY
The purpose of this review was to organize a systematic reference to help orthodontists evaluate commonly
used orthodontic nickel-titanium alloys. Part I of the article reviews the data available in the literature regarding
the temperature transitional ranges of the alloys. The thermomechanical behavior of these compounds is, in
fact, strictly dependent on the correlation between the temperature transitional range and the oral temperature
range. Part II of the article will focus on the mechanical characteristics of the alloys, such as the magnitude of
the forces delivered and its correlations with temperature transitional range and oral temperature. (Am J
Orthod Dentofacial Orthop 2001;119:587-93)
T
he engineering of nickel-titanium (NiTi) alloys
has made remarkable progress since the original
work of Buehler for the Naval Ordinance Laboratory in the early 1960s. Buehler’s preliminary results
led to development of the first NiTi orthodontic alloy
by pioneers such as Andreasen and his colleagues.1-3
New and improved materials are constantly being
proposed to the orthodontist, and this sometimes
increases confusion about the actual characteristics of
the wires. In fact, the ubiquitous claims of improved
performance are not always supported by correct information about the temperature transitional ranges
(TTRs) and the mechanical properties of the wire. Different parameters and experimental settings have been
used to analyze the performance of alloys with different compositions and properties, and several classifications have been suggested.4-9
Therefore, the aim of this review is to summarize
the criteria, validated by data available in the literature,
that the orthodontist might use to evaluate current commercial products.
A brief summary of the atomic composition and
thermomechanical behavior of NiTi alloys is provided
From the Division of Orthodontics at the Columbia University School of Dental and Oral Surgery, New York, NY.
aAssistant Professor of Clinical Orthodontics.
bAssociate Professor of Clinical Orthodontics.
cProfessor and Chairman.
Reprint requests to: Margherita Santoro, Division of Orthodontics, Columbia
University, School of Dental and Oral Surgery, 635 W 168th St, P O Box 20,
New York, NY 10032; e-mail, [email protected]
Submitted, January 2000; revised and accepted, April 2000.
Copyright © 2001 by the American Association of Orthodontists.
0889-5406/2001/$35.00 + 0 8/1/112446
doi:10.1067/mod.2001.112446
to clarify the theoretical basis of the inquiry. In addition, the importance of properly set TTRs is stressed.
MECHANICAL PROPERTIES AND COMPOSITION
OF NICKEL-TITANIUM ALLOYS
Thermoelasticity and shape memory effect
An ideal NiTi wire should retain a stable predesigned archform at mouth temperature and yet be
formable at a lower room temperature. In other words,
it should be possible to engage the wire into the brackets during a reasonable time interval, and only later
should the wire recover its ideal arch form and apply
light, predictable, constant, and continuous force to the
dentoalveolar structures.3 Because of the most recent
technological advancements, superelastic NiTi alloys
that meet these requirements are finally available.
Superelasticity is determined by the typical crystallographic characteristics of NiTi; the tri-dimensional lattice
of the alloy can be present in 2 phases: martensite and
austenite. In the martensitic phase, the lattice is bodycentered (cubic or tetragonal); in the austenitic phase, it is
face-centered (hexagonal close packed). An intermediate
rhombohedral “R” phase with a simple hexagonal lattice
has also been identified. In response to temperature variations, the crystal structure undergoes deformations in
which the molecular arrangement is modified without a
change of the atomic composition so that the transformation is diffusionless. The alloys essentially undergo a reorganization to meet the new environmental conditions—a
property that has earned them the designation of “smart
materials.”10-14 The transformation from the austenitic to
the martensitic phase (thermoelastic martensitic transformation) is reversible (pseudoshearing), whereas in metals
587
588 Santoro, Nicolay, and Cangialosi
Fig 1. Resistivity/temperature graph for superelastic
NiTi alloys. Mf, Martensite final; Ms, martensite start; As,
austenite start; Af, austenite final.
and other alloys an equivalent amount of stress usually
generates an irreversible shearing deformation.5,12-15
Each NiTi alloy has a specific temperature range in
which the phase transition takes place—the TTR. Ideally, the crystal structure of the alloys should be confirmed by means of either radiographic diffraction or
differential scanning calorimetry. However, because
austenite and martensite present different amounts of
resistance to the passage of electrical currents, it is possible to infer the phase transformation temperatures
through the study of resistivity.14,16,17
The diagram in Fig 1 illustrates the resistivity of a
superelastic NiTi alloy as it relates to the temperature
(resitivity/temperature curve). Ms and Mf are the initial
and the final temperatures, respectively, at which the
martensitic phase is formed; As and Af are the initial
and final temperatures for the austenitic phase.
At lower temperatures, the alloy is completely present in the martensitic phase (Mf to Ms) until the
increase in temperature causes the progressive transformation into austenite (from Ms to Af). At higher temperatures (beyond Af), the alloy exists exclusively in
the austenitic phase.12-14,18
An interesting feature of these thermoelastic properties is the so-called shape memory effect, which has
remarkable clinical applications. Through deflection and
repeated temperature cycles, the wire in the austenitic
phase is able to “memorize” a preformed shape, including specific orthodontic archforms. By lowering the temperature, the alloy is transformed into martensite and
becomes pliable and easily deformed. However, every
time the temperature rises above Af to the austenitic
phase, the wire will remember and recover the ideal archform. The technical name of the phenomenon is one-way
shape memory effect, since only 1 of the 2 phases, in this
case the austenite, retains a memorized shape.14,19
American Journal of Orthodontics and Dentofacial Orthopedics
June 2001
A practical explanatory example of the behavior is a
copper NiTi wire thermoactive at 40°C. At room temperature, when the wire is martensitic or in a mixed
phase, it is possible to introduce relatively sharp bends
in the wire. The original archform will be regained simply by heating the wire in hot water at a temperature
above Af, in this specific case, above 40°C.20
Shape memory alloys may fulfill the concept of the
ideal archwire, but, unfortunately, the presence of the
shape memory effect does not necessarily provide low
and continuous delivery of forces. In fact, for the memory property to be clinically detectable, the Af of the
alloy has to be set slightly below oral temperature so
that the wire will be primarily austenitic intraorally and
almost completely martensitic extraorally. However,
when the alloy is completely transformed into austenite
(temperatures above Af), the stress-strain curve follows
the regular pattern of other alloys, such as stainless
steel, with a direct proportionality between applied
stress and resulting strain and basically lacking the typical superelastic plateau. In other words, a NiTi alloy
completely transformed into austenite is definitely more
elastic than other alloys, but it is not “superelastic,” at
least in the absence of stress. The superelasticity of NiTi
wires is, in fact, correlated to the coexistence of the 2
phases.21 In Fig 1, in the section of the curve including
Ms and Af, the forces leading to a forward and a reverse
transformation between the 2 phases are equal. This
particular atomic equilibrium gives the NiTi lattice the
ability to absorb deflection stresses, and, as a consequence, the modulus of elasticity of the alloy becomes
typically low.12-14 As a general rule, the austenitic phase
of a superelastic wire will be stiffer than the martensitic
phase, but both will be stiffer than a superelastic wire in
phase transition. Therefore, one of the objectives of
research in the field should be the creation of new
alloys, such as copper NiTi 40°C, with TTRs that correspond to the oral environment temperature and reach
Af above the oral temperature. Unfortunately, the “natural charm” of the shape memory effect still holds the
interest of manufacturers and clinicians focused on
austenitic alloys with Af below oral temperature,
whereas characteristics such as magnitude and quality
of force delivery have somehow lost priority.
On the other hand, one may wonder if such a rigid
evaluation of the superelastic property of the alloys is
truly necessary in clinical applications. The performance
of any NiTi alloy is strictly influenced by its composition
and manufacturing procedures. For example, in theory, it
is possible that one specific alloy in its austenitic phase
(or even a work-hardened alloy) could deliver a biologically acceptable force, comparable to the force of a
superelastic alloy in phase transition, especially when
American Journal of Orthodontics and Dentofacial Orthopedics
Volume 119, Number 6
only moderate deflection is necessary. Despite commonly accepted commercial claims, the low values of
force delivery of most NiTi alloys are mainly hypothetical and still need to be precisely quantified and compared
with the force delivery of other established alloys through
properly designed experiments. Other factors, such as
stress, also have a strong influence on the phase transformation and, as a consequence, on the forces generated.
Pseudoelasticity
For most orthodontic alloys with an Af below oral
temperature, austenite is the prevalent phase intraorally,
while only a small percentage of martensite (and the
intermediate phase R) is present in the grain structure.
Because austenite has relatively higher stiffness compared with the transitional phase, greater quantities of
martensite need to form in these austenitic wires to activate the superelastic properties. Somehow, the martensite must form independently of temperature variations,
and, fortunately, some help in this regard comes from the
application of stress on the austenitic wire. The deflection generates a local martensitic transformation and
produces stress-induced martensite (SIM). The highest
temperature at which the martensite can form is referred
to as Md, and in austenitic alloys Md is usually located
above Af and above the oral temperature, allowing the
SIM to form in the stressed areas even if the rest of the
wire remains austenitic. However, the SIM is unstable,
and if the specimen is maintained at oral temperature it
undergoes reverse transformation to the austenitic phase
as soon as the stress is removed.14,22 In orthodontic clinical applications, SIM forms where the wire is tied to
brackets on misaligned teeth so that the wire becomes
noticeably pliable in the deflected areas, with seemingly
permanent deformation. Therefore, delivery forces will
be lowered in the needed areas only. In those areas, the
wire will be superelastic until, after tooth movement, a
self-controlled reduction of the deflection will restore
the stiffer austenitic phase. In summary, the formation of
SIM partially compensates for the lack of a thermally
induced martensite and contributes to the superelastic
behavior of austenitic NiTi alloys.
This property, termed pseudoelasticity, can be considered a localized stress-related superelastic phenomenon.23-29 However, for the SIM to form, the Af of the
alloy can only be slightly lower than the oral temperature.30 If Af is considerably lower than the oral temperature, the lattice will always have the tendency to
reconvert to austenite, and too much energy (in the
form of a deflective stress) would be necessary to counteract this tendency and maintain the presence of SIM.
Therefore, only in cases of very severe crowding will
an austenitic alloy behave superelastically.
Santoro, Nicolay, and Cangialosi 589
Before proceeding to the review of the properties of
the commercially available alloys, a few words are
needed on some recent investigations as to whether the
formation of SIM, as a modification of the lattice,
could interfere or modify the temperature range of the
phase transformation. Some recent investigations have
actually found that SIM generates a shift of the preset
TTR of the alloy toward higher temperatures because,
as a result of the presence of the mechanical deformation of the lattice, more energy will be required (in this
case in the form of heat) to reconvert SIM into austenite. This means that if the values of the TTR provided
by the manufacturers are not calculated under proper
conditions of deflection, those values might be underestimated and could fail to correspond to the actual
TTR values existing in orthodontic applications.21
In summary, the pseudoelastic (stress-related) and
thermoelastic (temperature-related) behaviors of NiTi
alloys are more complex and interdependent than
expected. The transformation temperatures provided
by the manufacturers, however, are still generally calculated in the absence of deflection, that is, in experimental settings that do not adequately replicate the
intraoral environment.
REVIEW OF AVAILABLE ALLOYS
Pseudoelastic and thermoelastic properties are not
always present in all commercial orthodontic alloys
available on the market. Sometimes the product advertisements can be confusing or even misleading. In this
review, we attempt to clarify a few points in this
regard, and, we hope, simplify the clinical choice by
attributing the correct properties to the most commonly used alloys.
NiTi orthodontic wires are generally classified as
superelastic or nonsuperelastic. The original Nitinol,
developed in the early 1960s, is a stabilized form of the
alloy in which work-hardening has abolished the phase
transformation. The alloy does not exhibit a shape
memory effect, but the low modulus of elasticity and
the high working range make Nitinol useful when considerable deflections are necessary.1-6,12,14,22
As a further development of Nitinol, the General
Research Institute for Non-ferrous Metals, in Beijing,
and the Furukawa Electric Co Ltd, in Japan, later proposed new NiTi alloys of similar composition, named,
respectively, Chinese NiTi and Japanese NiTi. The
more advanced alloys (now conventionally known as
Chinese NiTi) have different transition temperatures
than Nitinol and present a phase transformation. Chinese NiTi is austenitic at oral temperature, and,
because its TTR is lower than the oral temperature, it is
not intended to exhibit pure thermoelastic properties
590 Santoro, Nicolay, and Cangialosi
during clinical applications. On the other hand, Chinese NiTi does possess pseudoelastic characteristics
under stress because of the formation of SIM.31-34
The most recent technological advancements allow
the TTR to be set at specific temperatures (27°C, 35°C,
40°C), either through heat treatment and pressure variations (as with CV NiTi; Masel Industries, Bristol, Pa) or
through modification of atomic composition. In copper
NiTi, for example, the nickel in the alloy has been partially replaced by copper to produce ternary alloys.14,20
The classifications of NiTi compounds have evolved
with the introduction of new products. Waters’ review in
1992 divided the compounds into 3 groups, based on
their TTRs: Group 1 included alloys with TTRs between
room temperature and body temperature (martensitic
active alloys); group 2 included alloys with TTRs below
room temperature (austenitic); and group 3 included
alloys with TTRs close to body temperature, “which by
virtue of the shape memory effect spring back to their
original shape when activated by the body heat.”35
More recently, Evans and Durning36,37 introduced
an even more comprehensive classification of orthodontic alloys, dividing them into 5 groups: (1) phase I,
including alloys like gold and stainless steel, (2) phase
II, stabilized, (3) phase III, superelastic-active austenitic,
(4) phase IV, thermodynamic-active martensitic, and (5)
phase V, graded thermodynamic. The stabilized phase II
alloys correspond to the original work-hardened Nitinol, whereas the phase III active austenitic is predominantly austenitic at room temperature, with a TTR
below the intraoral temperature and thus presents pseudoelastic properties. The phase IV thermodynamic or
active martensitic alloys, because of the improved technical ability of dosing the amount of austenite present
in the alloys, have a TTR set very close to the intraoral
temperature or even corresponding to it. These alloys
have a great working range at room temperature;
because they exist in a mixed or rhombohedral phase at
room temperature, they are able to accept and maintain
seemingly permanent deformations. Once exposed to
the higher intraoral temperature, both the stressinduced and the temperature-dependent martensite will
be gradually converted into austenite, with a recovery
of the ideal preset archform and, at the same time, an
increase of force delivery.
Included in the active martensitic group are wires
with an Af set at a temperature above 37°C, such as
copper NiTi 40°C, which is almost completely transformed into martensite during clinical applications.
This wire is pliable both intraorally and extraorally, and
it will accept bends. The forces exerted on the dentoalveolar structures are remarkably low; therefore, the
alloy is recommended for the treatment of patients with
American Journal of Orthodontics and Dentofacial Orthopedics
June 2001
periodontal problems. The austenite will form only
when the temperature rises above 40°C. It is already
common practice to prescribe hot rinses to temporarily
increase force delivery to accelerate tooth movement.
Conversely, cold rinses will relieve discomfort.19
The low stiffness of copper NiTi 40°C also presents
the mechanical disadvantage of not allowing for complete dental alignment or full control of transverse
dimensions. A second wire with a larger diameter or
greater stiffness is usually required.
Temperature transitional ranges of available alloys
Several experiments have been performed in recent
years to verify whether the TTRs of orthodontic alloys
correspond to the values provided by the manufacturers. As previously mentioned, the TTR of the alloy
should be close to, or slightly below, the oral temperature to allow for the formation of SIM.
The intraoral temperature fluctuates widely
because of the ingestion of hot and cold food and beverages and because different areas of the oral cavity
exhibit different temperatures. Nevertheless, it can be
reasonably approximated in experimental settings
between 35°C and 37°C.38-40
Another source of variation in NiTi experiments is
the complexity of the manufacturing procedures. A
great variation in wire properties within batches is a
common finding that strongly limits the consistency of
the experimental data. Wires of similar composition
can show very different TTRs, especially when they
are manufactured by different companies. As a result,
the findings of different experiments are hardly comparable.28,35 Moreover, some manufacturers provide
accurate information about the TTRs of their NiTi
products, whereas others do not.
A review of the published data reveals that not all
of the NiTi wires available present clinically useful
thermal behavior (Table I).
Yoneyama, Doi, and Hamanaka,29 using differential
scanning calorimetry, found that most orthodontic wires
have TTRs between 17°C and 32°C. Practically speaking, such wires with the lowest Af are unable to exhibit
superelastic behavior during clinical applications.
Reflex wire (TP Orthodontics, La Porte, Ind) presents an Af set at 27°C, and 2 superelastic copper NiTi
wires (Copper Ni-Ti; Ormco, Orange, Calif) have Afs
set at 27°C and 35°C; such alloys can be safely classified as active austenitic. Heat Activated NiTi alloy
(Highland Metals, San Jose, Calif) has an Af set at
68°C and an Mf set at 24°C; this alloy has a considerably extended TTR, and the phase transition is present
consistently during clinical applications. 40°C
Thermo-Active Copper Ni-Ti (Ormco) can be consid-
Santoro, Nicolay, and Cangialosi 591
American Journal of Orthodontics and Dentofacial Orthopedics
Volume 119, Number 6
Table I. Transitional
temperature values of orthodontic NiTi alloys
Product
Manufacturer
TT unloaded
TT stress-related
Heat Activated NiTi
Thermomemoria
Reflex
Nitinol XL
27°C Superelastic
Copper Ni-Ti
35°C Thermo-Active
Copper Ni-Ti
40°C Thermo-Active
Copper Ni-Ti
Highland Metals
Leone
TP Orthodontics
3M Unitek
Ormco
24°C-68°C
Af 24°C
Af 27°C
—
—
—
Ormco
Af 33°C
Ormco
Af 39°C
Sentalloy
Neo Sentalloy
Nitinol Heat Activated
GAC
GAC
3M Unitek
Af 22°C
Af 28°C
Af 36°C
Af 31°C
ered truly superelastic at oral temperature. The Af of
Sentalloy (GAC International, Islandia, NY) is approximately 28°C in the absence of loading.21,25,27 Other
manufacturers generally claim an Af set at a nominal
oral temperature, approximately 35°C.12,17
Stress-related TTRs
It has been demonstrated for some NiTi alloys that
the formation of SIM generates a shift of the TTR
toward higher temperatures. This phenomenon is due
to the fact that the application of stress—in this case, in
the form of a deflection—maintains the atomic configuration of martensite at the expense of austenite.
Higher energy (heat, for example) will then be required
to reconvert the SIM into austenite. From a clinical
point of view, the stress-related TTR, or, more simply,
the stress-related Af, is the actual temperature at which
thermoactive wires will be “activated.” The stressrelated Af is, in fact, the temperature at which stressed
wires, tied to the brackets, will be transformed from the
low-stiffness martensite to the higher-stiffness austenite and will deliver a greater force to the dentoalveolar
stuctures. The immediate clinical advantage of the
stress-generated increase of Af is, of course, better
preservation of SIM and of pseudoelastic properties,
but the phenomenon also points out the necessity of
studying the TTR of orthodontic wires in the presence
of stress. Fortunately, the data available in the literature
are usually gathered through experiments that try to
reproduce a clinical setting and involve the application
of stress in various forms. Table I summarizes some
data on the TTRs of commercial orthodontic alloys.
Mullins, Bagby, and Norman17 analyzed Neo Sentalloy F100, F200, F300, and Bioforce Sentalloy
(GAC) with a deflection device that reproduced the
References
36, 37
36, 37
36, 37
3 brackets bending test
6-mm deflection Af 33°C
3 brackets bending test
6-mm deflection Af 39°C
21
3 brackets bending test
6-mm deflection Af 39°C
28°C
36°C
3 brackets bending test
6-mm deflection Af 38°C
21
21
22, 23
22, 23, 27, 41
21
loading of a maxillary lateral incisor. At 5°C, all of the
alloys were martensitic because they accepted a
pseudo-permanent deformation of 2.38 mm. However,
at 37°C, the permanent deformation under stress averaged only 0.08 mm, with no statistically significant differences between the alloys. The alloys were completely transformed into austenite at 37°C.17
Coluzzi et al27 compared the stress-related Af to the
Af of Neo Sentalloy F200 and Thermomemoria (Batch C
8922-16; Leone, Oxnard, Calif). The authors performed
a temperature cycle on the alloys with deflection and in
the absence of deflection. In the absence of deflection,
Neo Sentalloy exhibited an Af of 28°C. Af increased proportionally with loading to a maximum of 34°C. Leone’s
wire had an Af in unloaded conditions set at 20°C and
increased to a maximum of 35°C with loading.
Bishara et al41 compared the thermodynamic properties of Active Arch Nitinol (3M Unitek, Monrovia,
Calif), Heat-Activated Nitinol (Ortho Arch, Hoffman
Estates, Ill), and Neo Sentalloy. The experimental setting was a variation of a cantilever bend test in a temperature-controlled environment. Bends of 30° to 40°
were introduced in straight segments of the wires, and
the samples were then immersed in a water bath. All of
the samples showed 100% recovery of their original
shape. Active Arch Nitinol presented a TTR between
22.4°C and 28°C; Heat-Activated Nitinol had a TTR
between 23°C and 26.5°C, and Neo Sentalloy had a
TTR between 21°C and 28°C. Therefore, all the samples presented a TTR below oral temperature.41
In addition to Sentalloy and analogous austenitic
alloys, some of the new so-called thermoactive NiTi
alloys have also been studied under stress.
Santoro and Beshers21 tested Neo Sentalloy, 27°C
Superelastic Copper Ni-Ti (Ormco), 35°C and 40°C
592 Santoro, Nicolay, and Cangialosi
Thermo-Active Copper Ni-Ti, and Active Arch Nitinol
(3M Unitek). The experimental setting reproduced the
clinical loading of a crowded lower incisor. In agreement with previous studies, Neo Sentalloy was found
to be sensitive to loading, showing a stress-related Af
of 28°C, which agrees with previous studies. 27°C
Superelastic Copper Ni-Ti had a higher stress-related
Af of 32°C. Therefore, both the alloys were mainly
austenitic intraorally even in the presence of deflection.
35°C Thermo-Active Copper Ni-Ti had a stress-related
Af set at 39°C, while 40°C Thermo-Active Copper NiTi presented an Af above oral temperature; it was not
influenced by stress but rather displayed predictable
stress-related behavior. Thus, it can be considered a
true thermoelastic wire.21
SUMMARY AND CONCLUSIONS
Two concomitant phenomena are responsible for
the superelastic behavior of orthodontic NiTi alloys: a
temperature-related phase transformation along a TTR
(thermoelasticity), and the formation of SIM in localized areas of the archwire due to deflection (pseudoelasticity). The shape memory effect is derived from the
thermoelastic transformation from martensite to
austenite, and orthodontic clinical application requires
setting the TTR of the alloys slightly below oral temperature. This type of thermoelastic alloy, however,
will be completely austenitic at oral temperature, and
the austenite presents a higher modulus of elasticity
that results in a greater stiffness of the wire. In
austenitic alloys, the formation of SIM will guarantee
the presence of the superelastic behavior necessary for
the release of light and continuous forces.
Therefore, the Af of the alloy should not be set at a
temperature considerably below oral temperature or the
formation of SIM will not occur. It would actually be
advisable to evaluate alloys on the basis of their stressrelated TTRs because the application of stress usually
raises the Af of the alloy.
According to the data available in the literature,
most of the commercially available superelastic wires
exhibit stress-related Afs ranging from 22°C to 28°C,
and the TTRs of thermoelastic wires are set at higher
temperatures, from 35°C to 40°C.
In summary, 2 fundamental properties should be
taken into account to make an educated selection of a
NiTi wire: (1) a proper stress-related TTR, corresponding to or slightly below oral temperature and (2) a low
deactivation force released to the dentoalveolar structures to prevent deleterious side effects, such as pain
after bone hyalinization and possible root resorption.
This mechanical characteristic will be more extensively
considered in the second part of this review.
American Journal of Orthodontics and Dentofacial Orthopedics
June 2001
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